1. Field of the Invention
The present invention relates to porous layer open tubular (PLOT) columns for gas chromatography.
2. Background Art
Porous layer open tubular (PLOT) columns are useful for the analysis of volatile compounds. A PLOT column typically comprises a capillary column having a layer of a porous material coated on the inside surface of the columns. A PLOT column may contain a different type of coated materials to provide unique selectivity, allowing for the separation of gaseous compounds at room temperature. The retention times of compounds in PLOT columns depends partly on diffusion of the compounds into the thin porous layer and partly on the interaction between the compounds with the solid surfaces. Generally, gases and light hydrocarbons can be resolved at room temperature. In addition, PLOT columns can be heated to higher temperatures to elute higher boiling compounds.
The coatings of PLOT columns may contain various porous and non-porous materials, including alumina (aluminum oxide), molecular sieves, carbon based materials, and porous organic and inorganic polymers. PLOT columns are typically coated using a suspension of the porous material (e.g., colloidal alumina or polymer suspensions). The suspension medium is then removed leaving behind a stable uniform layer. Alternatively, the porous layer may be formed by polymerization in-situ. This layer of alumina or polymers may be further modified with various additives to give different selectivity.
A traditional PLOT column typically contains a layer of particles of 5-50 μm thick adhered to the inside tubing walls. The layers of porous materials are fragile due to the presence of particles and high porosity. The particles may come out of the column during use, especially when the tube is mechanically stressed or fast temperature changes take place. These particles dislodged from the columns may cause various problems. For example, detectors contaminated with particles typically generate electronic noise, which shows up as spikes in the baseline of the chromatogram. In extreme cases, detector flow can be obstructed by particle buildup. Particles can also affect valves by becoming lodged in the valve and causing leaks or restricting flow.
A possible solution for this problem can be an integrated porous plug as described in U.S. Pat. No 4,793,920. In principal this will work, because the porous plug will catch all the particles. However, another problem should occur, because all the particles will cumulate on one point, on the porous plug. This will clog the porous plug and will give a high pressure drop.
To alleviate these problems, PLOT columns may contain bonded stationary phases to provide higher mechanical and thermal stability. For example, U.S. Pat. Nos. 5,599,445, 5,607,580, 5,609,756, 5,620,603, and 5,630,937, all issued to Betz et al., disclose columns coated with a composition comprising a siloxane polymer having carbon bodies bonded thereto by direct carbon-to-silicon bonds. The columns are contacted with a mixture of the bodies and a hydrosiloxane polymer. The mixture is heated to cause the polymer to be bonded to the nucleophilic bodies, typically by C—Si, C—O—Si, Si—O—Si or Si—O—Al bonds, and to the column by reaction with the surface silanol or other nucleophilic groups.
Another example of a bonded stationary phase to provide a higher mechanical stability is the PoraBOND Q column (Chrompack/Varian). The coating of these columns is made in-situ to obtain a more homogeneous layer.
Even though the bonded particle approach minimizes the problems with particle leaks, it may not completely alleviate problems due to thermal expansion, which may cause the particles to break from the wall. An alternative approach is to use a particle trap. A particle trap is typically a piece of fused silica tubing coated with a reagent that can catch the released particles. Reagents that may be used in particle traps include siloxanes, which function as “glue” to bind particles dislodged from the PLOT columns.
The particle traps are typically coupled to the ends of PLOT columns by compression fits. These connector-particle trap configurations have two shortcomings: (i) the trap is typically coupled with a glass press fit connector, which may leak; and (ii) any particles dislodged from the column may be trapped at the connector, leading to clogging of the connector and a high pressure drop. Therefore, there remains a need for a better solution.